The Borg are not some unstoppable super-enemy, they are just Tough Cyborgs who are more advanced than the Federation. We've encountered several aliens throughout all of Trek that were more powerful than the Borg, so there's NO reason to treat them like something special.

Out of curiosity, what are they (leaving aside those species who have evolved beyond corporeal existence, like the Metrons, the Q, etc., species that wouldn't engage in armed combat)?